The present invention relates to a fixing belt, which can easily retain lubricant agent on its surface so as to reduce the abrasion resistance, a fixing device provided with the abovementioned fixing belt, and an image forming apparatus provided with the abovementioned fixing device.
In an image forming apparatus, which employs the electro-photographic method, such as a copier, a printer, a facsimile, a compound apparatus provided with such the functions, a latent image corresponding to the document image is formed on a photoreceptor drum, and a toner image is developed by applying toner onto the latent image and the developed toner image is transferred onto a recording medium. Then, the transferred toner image is fixed onto the recording medium to eject the recording medium with the fixed toner image outside the apparatus.
Further, when forming a full color image, four latent images of unicolor images Y (Yellow), M (Magenta), C (Cyan), K (Black), corresponding to the document color image, are formed on the four photoreceptor drums, respectively. Then, the four developed unicolor toner images are primary transferred onto an endless-type intermediate transfer belt, and then, secondary transferred onto the recording medium. Successively, the transferred full color toner image is fixed onto the recording medium to eject the recording medium with the fixed full color toner image outside the apparatus.
As the abovementioned fixing device for fixing a toner image, there has been employed a heat roller type fixing device, which applies heat and pressure to the recording medium on which a toner image is already transferred, while tightly clipping and conveying the recording medium at a nip portion formed by making a fixing roller incorporating a heating element, such as a halogen lamp, etc., and a pressure roller for applying a pressure onto the fixing roller, press-contact each other. Since the structure of the fixing device of this type is relatively simple, such the fixing device has been widely employed in various kinds of image forming apparatuses.
Incidentally, in order to increase the fixing velocity of such the fixing device as mentioned in the above, it is necessary to supply a sufficient amount of heat to both the toner and the recording medium, and therefore, to widen the width of the nip portion. In order to widen the width of the nip portion, it could be considered to increase the pressing load applied to the fixing roller by the pressure roller, a thickness of the elastic layer made of silicon rubber, etc., formed on the pressure roller, or the diameters of both the fixing roller and the pressure roller.
However, when the pressing load or the thickness of the elastic layer of the pressure roller is increased, sometimes, the width of the nip portion in the axis direction becomes uneven, and this would possibly cause a fixing unevenness or a wrinkle of the recording medium. Further, when the diameters of the rollers are increased, there has been a problem that the fixing device not only is getting large-sized, but also requires a longer warming up time than ever.
To overcome the abovementioned problem, Patent Documents 1 and 2 (Tokkai 2005-173441, Tokkai 2001-341143, both are Japanese Non-Examined Patent Publications) set forth a fixing device, which is provided with: a fixing roller having an elastic layer made of a silicon rubber and rotating with a heating element, such as a halogen lamp, etc., incorporated at the center axial position of the fixing roller; an endless-type fixing belt circulating and driven by the fixing roller; and a pressing pad disposed at an inner circumferential surface side of the endless-type fixing belt, so as to press the endless-type fixing belt towards the fixing roller by means of the pressing pad.
According to the fixing device set forth in Patent Documents 1 and 2, the elastic deformation of the endless-type fixing belt pressed onto the fixing roller by the pressing pad creates a nip portion having a wide width between the fixing roller and the endless-type fixing belt. Accordingly, it becomes possible to cope with the high speed trend of the fixing device without increasing the size of the fixing device. Further, since the heat capacity of the endless-type fixing belt is relatively small, it becomes possible to shorten the warming up time, resulting in energy save.
In the fixing device set forth in Patent Documents 1 and 2, since the inner circumferential surface of the endless-type fixing belt rubs the fixed pressing pad while circulating, if no countermeasure is applied, there is a fear that deterioration of the fixed image quality due to the deviation of the endless-type fixing belt, increase of the power consumption of the motor for driving the endless-type fixing belt due to the increase of the driving torque of the endless-type fixing belt, failure of the speed reducing gear, abrasion of the inner circumferential surface of the endless-type fixing belt, etc. would occur, due to the large amount of the abrasion resistance between them. Accordingly, in order to reduce the abrasion resistance between the inner circumferential surface of the endless-type fixing belt and the fixed pressing pad, the fixing device is provided with a lubricant supplying member, which is made of sponge, etc. and press-contacts the inner circumferential surface of the endless-type fixing belt so as to supply lubricant agent on it. Further, an rubbing member, made of a low frictional material, is disposed between the endless-type fixing belt and the pressing pad.
However, even in the configuration mentioned in the above, since it is difficult for the inner circumferential surface of the endless-type fixing belt to stably retain the lubricant agent on it, depressions for retaining the lubricant agent in them, namely, appropriate degree of unevenness, are required to be formed on the inner circumferential surface of the endless-type fixing belt. Accordingly, in the fixing device set forth in Patent Document 1, the surface roughness of the inner circumferential surface of the endless-type fixing belt is defined by the arithmetical mean deviation of the profile (Ra), while, in the fixing device set forth in Patent Document 2, that is defined by the ten point height of irregularities (Rz).
Incidentally, the arithmetical mean deviation of the profile (Ra) can be derived from the steps of: extracting only the reference lengths from the roughness profile; totaling absolute values of the deviations from the average lines at the extracted portions to the measuring profile; and finding the arithmetical mean deviation by averaging the total absolute values. On the other hand, the ten point height of irregularities (Rz) can be derived from the steps of: extracting only the reference lengths from the roughness profile; averaging the absolute values of the peak heights of the five mountaintops from the highest to the fifth among all mountaintops from the average profile of the extracted portions; averaging the absolute values of the peak heights of the five mountaintops from the lowest to the fifth among all mountaintops from the average profile of the extracted portions; and calculating the ten point height of irregularities by adding the former average value and the latter average value to each other. The definitions of the abovementioned calculation procedures are detailed in the JIS B0601 (Japanese Industrial Standard B0601).
On the other hand, the present inventors have been intensively studied the relationship between the surface roughness of the inner circumferential surface of the fixing belt and the quality improvement of the fixed image and the reduction of the driving torque for the fixing belt. As a result of such the studies, the present inventors have found that the aforementioned definitions for the surface roughness are insufficient. Concretely speaking, even if the same value of the arithmetical mean deviation of the profile (Ra) is obtained with respect to different measurement cases, sometimes, the image quality and the driving torque varies in either a good case or a bad case, depending on the unevenness variation of the surface roughness. Accordingly, the present inventors have found that it is not a secure countermeasure to define the surface roughness of the inner circumferential surface of the fixing belt by employing the arithmetical mean deviation of the profile (Ra).
In other words, the arithmetical mean deviation of the profile (Ra) and the ten point height of irregularities (Rz) are derived by simply averaging heights of microscopic protrusions and depths of microscopic depressions residing on the surface, without taking expanses of the microscopic protrusions and the microscopic depressions into account. Accordingly, at a microscopic depression, since the lubricant agent protruded from the microscopic depression due to the interfacial force, it is difficult to securely retain the lubricant agent within the microscopic depression. Therefore, even if the arithmetical mean deviation of the profile (Ra) is the same as that in the good case, when the surface includes a lot of excessive microscopic depressions, it is difficult to retain the lubricant agent on the surface, and accordingly, the image quality and the driving torque are getting worse.